Leukemias and lymphomas are attractive targets for treatment with immunotoxins. The response of patients with B-cell malignancies has been extensively investigated in phase I/II clinical trials of immunotoxin activity. Amlot et al., (1993), Blood 82, 2624-2633; Sausville et al., (1995) Blood 85, 3457-3465; Grossbard et al., (1993) Blood 81, 2263-2271; Grossbard et al., (1993) Clin. Oncol. 11, 726-737. To date, some antitumor responses have been noted but immunotoxin mediated toxicity to normal tissue often prevented dose escalations to therapeutic levels. Several B-cell-specific antigens such as CD19, CD22 and CD40 have been targeted by immunotoxins made with plant toxins such as ricin A-chain and bacterial toxins, such as Pseudomonas exotoxin A (PE). Uckun et al., (1992), Blood 79, 2201-2214; Ghetie et al., (1991), Cancer Res. 51, 5876-5880; Francisco et al., (1995), Cancer Res. 55, 3099-3104.
CD22, a lineage-restricted B-cell antigen that belongs to the Ig superfamily, is expressed on the surface of many types of malignant B cells, including chronic B-lymphocytic cells (B-CLL), B lymphoma cells such as Burkitt's lymphomas, and hairy cell leukemias, as well as on normal mature B lymphocytes. CD22 is not present on the cell surface in early stages of B-cell development, and is not expressed on stem cells. Vaickus et al., (1991), Crit. Rev. Oncol/Hematol. 11, 267-297. Additionally, no shed antigen can be detected in normal human serum or serum from patients with CLL. Li et al., (1989), Cell. Immunol. 118, 85-99.
RFB4 IgG is an anti-CD22 monoclonal antibody. This antibody has been chemically conjugated to both ricin and Pseudomonas exotoxin A (PE) and has shown activity against B-cells both in vitro and in vivo; Ghetie et al., (1991), Cancer Res. 51, 5876-5880; Ghetie et al., (1988), Cancer Res. 48, 2610-2617. RFB4 is highly specific for cells of the B lineage and has no detectable cross-reactivity with any other normal cell type. Li et al., (1989), Cell. immunol 118, 85-99. RFB4 IgG has previously been covalently coupled to both ricin A-chain and a truncated form of PE called PE35. These conjugate molecules were effective against experimental lymphoma xenograft models, and in the clinical setting, the ricin-based immunotoxin also has shown some efficacy against human disease. Amlot et al., (1993), Blood 82, 2624-2633; Sausville, (1995), Blood 85, 3457-3465.
While chemical conjugates are frequently very stable and potent, they are large and presumably heterogeneous at their linkage sites, which may result in sub-optimal activity. The requirements for making large quantities of IgG and chemical conjugation also put some limitations on the ability to manufacture the drug. Because the ability to penetrate tumors is inversely related to the size of the penetrating molecule, the large size of antibody-toxin conjugates may impair their ability to penetrate tumor masses such as those found in lymphomas.